Method and apparatus for the concentration of solutions

ABSTRACT

A feed solution containing a solute and a solvent is concentrated by reverse osmosis. The feed solution is subjected to a given working pressure and is first forced through a series of high rejection membranes. The diluted solution which permeates the high rejection membranes is discarded and the product solution leaving the high rejection membranes is then passed as a feed solution through a series of low rejection membranes. The dilute solution which permeates through the low rejection membranes is preferably recycled to the starting feed solution, but can be discarded, if desired. The product comprises the concentrated solution which does not permeate the low rejection membranes. The working pressure of the solution fed to both the high rejection membrane and the low rejection membranes should exceed R2 pi &#39;&#39; where: R is the rejection ratio of the membrane and pi &#39;&#39; is the osmotic pressure of the solution being fed to the membrane.

United States Patent lnventors Chaim Elata Savlon; Jonatan Ben-Dror, Rehovot, both of Israel [21] Appl. No. 864,309 [22] Filed Oct. 7, 1969 [45] Patented Nov. 2, 1971 [73] Assignee Hydronautics-Israel Ltd.

Rehovot, Israel [32] Priority Apr. 4, 1969 [33] Israel [31] 31963 [54] METHOD AND APPARATUS FOR THE CONCENTRATION OF SOLUTIONS 7 Claims, 1 Drawing Fig.

[52] US. Cl 210/23, 210/321 [51] Int.Cl 801d 13/00 [50] Field ofSearch 210/22, 23, 321, 500, 252, 259

[56] References Cited UNITED STATES PATENTS 2,540,152 2/1951 Weller 210/500 X OTHER REFERENCES Michaels, Ultrafiltration, a Publication of the Amicon Corporation, March, 1968, 27 pages, pp. 13-25 relied on.

Primary Examiner-Frank A. Spear, Jr. Attorney-Finnegan, Henderson & Farahow ABSTRACT: A feed solution containing a solute and a solvent is concentrated by reverse osmosis. The feed solution is subjected to a given working pressure and is first forced through a series of high rejection membranes. The diluted solution which permeates the high rejection membranes is discarded and the product solution leaving the high rejection membranes is then passed as a feed solution through a series of low rejection membranes. The dilute solution which permeates through the low rejection membranes is preferably recycled to the starting feed solution, but can be discarded, if desired. The product comprises the concentrated solution which does not permeate the low rejection membranes. The working pressure of the solution fed to both the high rejection membrane and the low rejection membranes should exceed R1r' where: R is the rejection ratio of the membrane and 1r is the osmotic pressure of the solution being fed to the membrane.

PATENTEUNUV 2 I971 INVENTORS CHAIM ELATA JONATAN BEN DRO R ATTORNEYS METHOD AND APPARATUS FOR THE CONCENTRATION OF SOLUTIONS The present invention relates to a new and improved system for the concentration of solutions, and more particularly, to a new and improved method and apparatus for concentrating solutions by reverse osmosis.

Various techniques for concentrating solutions by removal of the solvent are well known and include such methods as evaporating the solvent by heating or in a vacuum or freezing the solvent and then removing it in crystallized form.

Such methods however have not been entirely satisfactory in the past because they are based on a change of phase to effect solvent removal and are oftentimes complicated and expensive to carry out. Further, some of the dissolved solids which should be retained in the concentrated solutions are lost and/or decomposed by these methods.

More recently, reverse osmosis techniques have been tired in an attempt to overcome many of the problems of conventional liquid concentration procedures.

Briefly, the principles of reverse osmosis can be summarized as follows: When a solution and pure water are separated by a barrier (membrane) which is permeable to water, an energy gradient is generated which causes the water to pass through the membrane from the pure water side to the solution.

The hydraulic flow rate of the water can be related to an effective pressure head which is proportional to the concentration of the solution and is called the osmotic pressure of the solution. Hence, hydraulic flow can be slowed down by increasing the hydrostatic head on the solution side. When the hydrostatic head becomes equal to the osmotic pressure, no flow occurs, and when the hydrostatic head exceeds the osmotic pressure, the flow is reversed and the water now passes from the solution to the fresh water side of the separating membrane.

In the specification and claims of this patent application, the solution to be concentrated containing a solvent and a solute will be called a feed solution" and the concentrated solution obtained during the concentration process will be called product solution."

Hence, in concentrating solutions by reverse osmosis, the feed solution is placed in contact with a semipermeable membrane, i.e., a membrane through which water may flow but which is s substantially impervious to any solutes such as sugars, organic esters and electrolytes, and a pressure is applied to the feed solution, which is larger than its osmotic pressure, Thus, relatively pure water flows through the membrane and a solution enriched in solute remains on the feed solution side of the membrane.

The above explanation has been made with reference to water, however, a variety of solvents can be removed using reverse osmosis concentration techniques.

While such reverse osmosis techniques have been used experimentally for concentrating solutions, serious difliculties have been encountered in attempting to commercially concentrate solutions by reverse osmosis.

A concentrated product solution usually has a high osmotic pressure. Thus, very high hydrostatic pressures that exceed the osmotic pressure of the product solution must be applied in order to obtain concentrated products. Presently available membranes, however, undergo rapid compaction when subjected to such high pressures and suffer a simultaneous loss in flux (permeability rate). This compaction reduces the useful life of the membrane and can significantly affect the cost of concentrating solutions if frequent replacement of membranes is made necessary. A related problem is the capital expense required to construct an entire system capable of withstanding the high pressures.

Thus, there has been a continuing search to find a method based on the principles of reverse osmosis, which can economically produce concentrated solutions at reduced hydrostatic pressures and thereby permit the successful use of commercially available membranes and the use of relatively inexpensive apparatus.

The present invention is based on a method for the concentration of a feed solution by way of reverse osmosis in which the product solution produced by the method is characterized by an osmotic pressure that is higher than the sum of the highest hydrostatic working pressure applied to the feed solution and the osmotic pressure of the solution that permeates through the reverse osmosis membranes and is discarded.

The invention also provides a method in which the volumetric flows are minimized so that only a relatively small area of membrane is needed to produce a desired volume of product per day.

in accordance with the present invention, a feed solution to be concentrated is passed in succession through two series of membranes; the first series of membranes being composed of high rejection membranes and the second series of membranes being composed of high rejection membranes.

The present invention also comprises an apparatus including a high rejection membrane or preferably a plurality of high rejection membranes in series which first received the feed solution and separate the feed solution into a concentrated feed solution which does not pass through the membranes and a solvent solution which permeates through the membranes; means for passing the concentrated feed solution to a plurality of low rejection membranes which separate the concentrated feed solution into a product solution which does not pass through the second stage of low rejection membranes and a dilute solution which permeates through the low rejection membranes.

The apparatus also includes means for applying and adjusting the working pressure within the membranes; supply means for feeding the feed solution to the high rejection membranes; removal means for disposing of the solvent solution that permeates through the high rejection membranes; and means for collecting and withdrawing the concentrated product solution that has not passed through the low rejection membranes.

Means are also provided for either removing and disposing of the dilute solution that permeates through the low rejection membranes or for recycling it to the feed solution.

It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory, but are not restrictive of the invention.

The accompanying drawing, which is incorporated in the constitutes a part of this specification, illustrates an embodiment of the invention and together with the description serves to explain the principles of the invention.

The drawing is a schematic flow sheet of an apparatus for carrying out the method of the invention.

The performance of the method according to the present invention may be best explained by reference to the following equations:

The salt rejection R in reverse osmosis is given by the formula R=C'C"/C'=lC"/C (l wherein C and C" refer to the solid concentrations on the high-pressure and low-pressure side of the membrane, respectively. Hence the diflerence AC between the concentrations can be expressed by the formula:

AC=C"C"=RC' (2) The osmotic pressure 11' of a solution is nearly directly proportional to its concentration and thus the osmotic pressure difference Arr can be expressed by the fonnula: Atr=-Rw'.

Furthermore: Jv=-Lp(AP-0A1r) (3) wherein:

-Jv water flux, i.e., the number of gallons of solvent, for example, water that pass through i square foot of the membrane per day;

Ap hydrostatic pressure difierence between the low-pressure and the high-pressure sides of the membrane;

Lp volume permeability coefficient, i.e., number of gallons of solution permeating through 1 square foot per day under a pressure of l p.s.i.;

A1r= osmotic pressure difference between the solutions at the low-pressure and the high-pressure sides of the membrane; and 0=salt rejection at infinite applied pressure difference;

When Jv is large, 6E=R and thus equation (3) is converted to Jv=-Lp(AP-l-R 1r') (4) For reverse osmosis to occur, therefor, the hydrostatic pressure applied to a given membrane must always be larger than system. This same arrangement also forms a preferred low rejection membrane system if large volumes of solvent are to be separated from the solution in the second stage of the process. Usually, the membrane will be externally supported to R'1r'. 5 withstand the working pressures involved in reverse osmosis The present invention thus comprises a method for concenoperations. The dimensions of the membranes and chosen to trating a feed solution wherein the feed solution while being reflect the volume and degree of concentration desired for the subjected to a certain working pressure is first passed through specific feed solution to be concentrated. aseries of high rejection membranes. The diluted solution per- The method of the present invention can be used for the meating through the membranes is discarded and the product concentration of any feed solution to a product solution havsolution leaving the high rejection membrane is, while still ing an osmotic pressure which is larger than the highest being subjected to a working pressure, passed as feed solution required working pressure. Typical feed solutions include for through a series of low rejection membranes. example, fruit juices, such as orange juice, strawberry juice,

The product solution is collected and the diluted solution and apple uice, and dilute sugar solutions. permeating through the low rejection membranes can be The method of the invention can be performed batchwise or discarded,or preferably, recycled to the starting feed solution. continuously, the latter being preferred. As the solids per- The working Pressure of the Solution fed t0 the g rejection meating as part of the diluted solution through the low rejecmemblahes Should be larger than thevahle of R211" tion membranes are very often valuable ingredients it is adbefote defined) for the Sh teleeheh 'e and the vantageous and desirable to recycle this diluted solution to the wolklhg Pressure of the sohltloh fed to the ltzwfeleetloh them original feed solution. Because only a small fraction of the init h Should be larger than the valve of R for the low tial feed is recycled, the area of membrane required to prac- Jeettoh e h e tice the process is kept at a minimum.

te-lect'vtty of the h teletfttoh methbtahes t be An embodiment of an apparatus according to the present t than that of the tohowthg sehes 0t t h h invention is illustrated inthe accompanying drawing. The vari- Jecttoh memthahes are those thembranles m wh'ch a maxlmum ous parts of the apparatus can be constructed from a variety of h z h i healt y percent t materials that are compatible with the various ingredients of on r g, pure zigg gia the solution and that can withstand the working pressures. For rqlectloh g rams n z d e ee processing food solutions, stainless steel is usually preferred. so i g h 1 e f c The apparatus comprises a feed tank 10, a high pressure .Sute o t e pro so near y equa S e wor pump 12, a high rejection membrane system 14, a low rejecing pressure applied.

tion membrane system 16, a product tank T8, and a water tank Low re ection membranes allow some dissolved solids to 20. The pressures within the membrane systems 14 and 16 are permeate through the membrane with the solvent. in this case t d b 22 d 24 l R l and the feed solution, which has been already concen- :de il lh zz Jus m3 g gf ah I lr ytrated to a certain extent in the first stage, i.e., within the high f h as me are 6 a {f' t e ute rejection membranes, can be concentrated until the new Rqr' tom System 16 to tank 1 elves e also P of the low rejection membranes again neafly equals the work vided for controlling the flow of the solutions at various stages ing pressure in the apparatus and an air accumulator 27 IS positioned ust The solids permeability rate'the low rejection membrane 40 t P can vary depending on the working conditions and concentra- [h opetattohi the t h sohmoh Pumped by P tion of solution desired. The lower the solids concentration tank 10 to a high teleettoh membrane y e 14, whel'elh P requirement for the product solution, for example, the higher of the Solvent, -B-i Water, penetrates Practically P through can be the rejection rate f the membranes because h the membrane to water tank 20, while the concentrated solupmduct l ti ill h a l ti pressure d tion flows to low rejection membrane system 16. A diluted reasonable hydrostatic pressure in the second stage will still solution penetrates through the membranes of system 16 and produce acceptable flux rates. is recycled to feed tank 10 via line 26. The product solution On the other hand, the higher the required concentration of from low rejection membrane system 16 is collected in tank the final product solution (and thus its osmotic pressure), the 18. When the process is performed continuously the feed solumore permeable the low rejection membranes should be. tion is continuously added to tank 10 through line 28 and the Preferably, the amount of dissolved solids retained should at r du t l ti in tank 18 and the olv t i t k 20 are conleast equal the amount of solids permeating through the ti l removed. second stage membrane with the sol e t. The following examples are illustrative of the invention but The yp of membrane to be utilized is not a critical feature are not to be understood as limiting the scope and underlying of e Present ihvehtiohihs 10118 as the membl'ahe fulfills the principles of the invention in anyway. All parts and percentl'equh'emehts of the speelhe eheehtmhh 'ee to be P ages referred to herein are by weight unless otherwise specififormed, i.e., has the desired volume piermeallz hty coefficient Cally indicated (Lp) and withstands the require wor mg pressures. Moreover, the membrane should be insoluble and chemically EXAMPLES inert in relation to the solvent and all the ingredients of the In these mpl cellulose acetate m m r w e solution. Cellulose acetate membranes, which are commerin the apparatus described in the accompanying drawing and cially available, a presently preferred. The membranes may pressure and concentration measurements were performed at have any suitable shape, for example, the membranes may be the points marked on the drawing. tubular or be flat. A series of tubular membranes having their The results of the concentration, pressure, and flux meaends connected forms a preferred high rejection membrane surements are shown in table 1 below, where:

Salt rejrectlon 0 Concentrations Flow rates membranes Watt-r flux B o) (at 300 c (liters/day) Pressures (p.s.i.) (percent) (g.f.d.) Example Feed solution CF C0 C1 s Q1" Q0 Q1 Q2 Q2 o PA P1 PB R R Jv ;v

1) Sugar solution. 10. 3 10.6 0 15 44. 5 148 161 112 13 36 830 820 810 790 760 100 63 0. 35 0.1 2).. Orange Juice.. 10. a 10. 5 0 7 37 128 139 11 33 800 850 820 740 700 77 0.725 0.1 3).. "do 10.9 10. 5 0 6 36.5 161 174 13 46 900 860 2110 750 670 100 88 0. 88 0.1 (4) ,do 10 5 l3. 0 0 19. 5 46. 5 102 79 43 23 8 0 800 680 600 100 50 50 0. 66

C= concentration in degrees Brix (Bf) measur ed at 30 C.

Q rate of flow of the solution in liters per day. Jv water flux expressed as gallons of water that pass through 1 sq. foot of membrane per day g.f.d. R average rejection rate of the membrane. P= working pressure in p.s.i. Referring more specifically to the apparatus used in these examples, the high pressure series of membranes 14 comprises 36 tubular h inches inside diameter membranes aligned in series and the low-pressure membrane 16 also comprises 36 tubular 1% inches diameter membranes.

The effectiveness of the new method and apparatus is readily understood from the results of the Examples. The invention permits concentrating feed solutions to obtain a product solution which has a higher osmotic pressure than the sum of the highest hydrostatic working pressure applied to the solution,

and the osmotic pressure of the solvent solution which is discharged from the system.

Further, the invention permits operating at both desirable pressure levels to avoid membrane compaction and a desirably low ration of volume of solvent passed through the membranes per volume of product recovered. The latter characteristic helps reduce the area of membrane required to obtain apparatus having a given product capacity.

What is claimed is:

l. A method for concentrating a feed solution comprising: forcing a feed solution through a high rejection membrane by subjecting the feed solution to a first working pressure, the diluted solution permeating through the membranes being discarded; forcing the concentrated solution which does not pass through the high rejection membrane and which is still being subjected to a second working pressure as a feed solution to a second membrane having a solid rejection rate that is lower than the rejection rate of the high rejection membrane;

and collecting as a product solution the portion of the feed to the second membrane which does not pass through the second membrane, the first and second working pressures being greater than R 'rr' where R is the rejection rate of the respective membrane, and 1r is the osmotic pressure of the solution being fed to the membrane.

2. The method of claim 1, in which the dilute solution which passes through the second series of membranes is recycled to the high rejection membrane as part of the feed solution,

3. The method of claim 2 wherein the feed solution is a sugar solution.

4. The method of claim 2 wherein the feed solution is a fruit juice solution.

5. The method of claim 1 wherein the membrane is a cellulose acetate membrane.

6. An apparatus for concentrating a feed solution comprising: a high rejection membrane which receives the feed solution and separates the feed solution into a concentrated feed solution which does not pass through the membrane, a low rejection membrane which receives the concentrated feed solution that does not pass through the high rejection membrane and separates the concentrated feed solution into a product solution which does not pass through the second membrane, and a diluted solution which does not pass through the second membrane; means for applying and for adjusting the working pressure within said membranes; supply means for forwarding the feed solution to said high rejection membrane; means for passing the concentrated feed solution to the low rejection membrane product collection means connected to the high pressure side of said low rejection membranes; and removal means for discharging the diluted solution which penneates through the high rejection membranes.

7. The apparatus of claim 6 including means for recycling to the supply means the solution permeating through the low rejection membranes to prevent loss of valuable dissolved solids.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Dated November 2, 1971 Patent No. 3, 617, 550

Inventor(s) Chaim Elata and Jonatan Ben-Dror It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Claim 6, line 9, after which does delete "not".

(SEAL) Attest:

EDWARD I LFLETCHEILJR. ROBERT GOTISCHALK Attesting Officer Commissioner of Patents FORM po'mso (1069) USCOMM-OC 60376-P69 U 5 GOVERNMENT PRINT NG OFFICE H59 0-356-53 

2. The method of claim 1, in which the dilute solution which passes through the second series of membranes is recycled to the high rejection membrane as part of the feed solution,
 3. The method of claim 2 wherein the feed solution is a sugar solution.
 4. The method of claim 2 wherein the feed solution is a fruit juice solution.
 5. The method of claim 1 wherein the membrane is a cellulose acetate membrane.
 6. An apparatus for concentrating a feed solution comprising: A high rejection membrane which receives the feed solution and separates the feed solution into a concentrated feed solution which does not pass through the membrane, a low rejection membrane which receives the concentrated feed solution that does not pass through the high rejection membrane and separates the concentrated feed solution into a product solution which does not pass through the second membrane, and a diluted solution which does not pass through the second membrane; means for applying and for adjusting the working pressure within said membranes; supply means for forwarding the feed solution to said high rejection membrane; means for passing the concentrated feed solution to the low rejection membrane product collection means connected to the high pressure side of said low rejection membranes; and removal means for discharging the diluted solution which permeates through the high rejection membranes.
 7. The apparatus of claim 6 including means for recycling to the supply means the solution permeating through the low rejection membranes to prevent loss of valuable dissolved solids. 